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Two-stage compressor refrigeration principle

The two-stage compressor refrigeration cycle generally uses two compressors, namely a low-pressure compressor and a high-pressure compressor.

1.1 The process of refrigerant gas increasing from evaporating pressure to condensing pressure is divided into 2 stages

The first stage: Compressed to the intermediate pressure by the low-pressure stage compressor first:
The second stage: the gas under the intermediate pressure is further compressed to the condensation pressure by the high-pressure compressor after intermediate cooling, and the reciprocating cycle completes a refrigeration process.

When producing low temperatures, the intercooler of the two-stage compression refrigeration cycle reduces the inlet temperature of the refrigerant in the high-pressure stage compressor, and also reduces the discharge temperature of the same compressor.

Since the two-stage compression refrigeration cycle divides the entire refrigeration process into two stages, the compression ratio of each stage will be much lower than that of single-stage compression, reducing the requirements for equipment strength and greatly improving the efficiency of the refrigeration cycle. The two-stage compression refrigeration cycle is divided into an intermediate complete cooling cycle and an intermediate incomplete cooling cycle according to the different intermediate cooling methods; if it is based on the throttling method, it can be divided into a first-stage throttling cycle and a second-stage throttling cycle.
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1.2 Two-stage compression refrigerant types

Most of the two-stage compression refrigeration systems choose medium and low temperature refrigerants. Experimental research shows that R448A and R455a are good substitutes for R404A in terms of energy efficiency. Compared with alternatives to hydrofluorocarbons, CO2, as an environmentally friendly working fluid, is a potential substitute for hydrofluorocarbon refrigerants and has good environmental characteristics.

But replacing R134a with CO2 will deteriorate the system performance, especially at higher ambient temperatures, the pressure of the CO2 system is quite high and requires special treatment of key components, especially the compressor.

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1.3 Optimization research on two-stage compression refrigeration

At present, the optimization research results of the two-stage compression refrigeration cycle system are mainly as follows:
(1) While increasing the number of tube rows in the intercooler, reducing the number of tube rows in the air cooler can increase the heat exchange area of ​​the intercooler while reducing the air flow caused by the large number of tube rows in the air cooler. Returning to its inlet, through the above improvements, the inlet temperature of the intercooler can be reduced by about 2°C, and at the same time, the cooling effect of the air cooler can be guaranteed.

(2) Keep the frequency of the low-pressure compressor constant, and change the frequency of the high-pressure compressor, thereby changing the ratio of the gas delivery volume of the high-pressure compressor. When the evaporation temperature is constant at -20°C, the maximum COP is 3.374, and the maximum The gas delivery ratio corresponding to COP is 1.819.

(3) By comparing several common CO2 transcritical two-stage compression refrigeration systems, it is concluded that the outlet temperature of the gas cooler and the efficiency of the low-pressure stage compressor have a great influence on the cycle at a given pressure, so if you want to To improve system efficiency, it is necessary to reduce the outlet temperature of the gas cooler and select a low-pressure stage compressor with high operating efficiency.


Post time: Mar-22-2023